1. Field
The embodiment relates to a multiple-output DC/DC converter and a power supply having the same.
2. Background
There is a need to boost a voltage in order to supply energy to an electronic circuit requiring a relatively high voltage, e.g., use a voltage for applications in connection with a power system. It may be also necessary to decrease a voltage from a high voltage to a low voltage according to an electronic circuit. To this end, the study of modeling and analyzing a DC/DC converter as one among various voltage boosting and dropping converters has been performed. The DC/DC converters may be classified into an insulation type and a non-insulation type.
The input and output of the insulation type converter may be insulated by using a transformer having a magnetic core, so that stability is secured. The voltage boosting and dropping ratios of the insulation type converter may be adjusted by adjusting a turn ratio.
The insulation type converters may be classified into a buck type and a buck-boost type. The buck type converter includes a forward converter, a half bridge converter and a full bridge converter. The buck-boost type converter includes a flyback converter. Particularly, since the flyback converter is operated even with only one switching device, the flyback converter may be implemented at a low cost.
FIG. 1 shows a conventional power supply circuit having a DC/DC converter. The DC/DC converter 20 of the power supply circuit 10 may receive the DC power generated by full-rectifying AC power through a rectifier (not shown) as input power Vi. The DC power may be generated by allowing the full-rectified signal to pass through a filter or an additional circuit for improving a power factor.
The DC/DC converter 20 may include at least one switch device and the output voltage of the secondary side of the DC/DC converter 20 may be determined to have a desired value according to a switching scheme of the switch device. A controller 30 controls the on/off operation of the switch to output a desired voltage Vo from secondary side. The controller 30 may include a PWM control unit and a driving unit.
In general, the power supply 10 includes a voltage-current sensing unit 40 which senses a current or voltage of the secondary side or all the current and voltage of the secondary side to feedback the current or voltage or all the current and voltage of the secondary side to the controller, such that the output voltage Vo of the secondary side is controlled. In case of current sensing, since the primary side and the secondary side are insulated from each other, current may be feedbacked using an optocoupler device for transmitting a signal as light.
As one example, a case that a flyback converter serves as the DC/DC converter 20 will be described. While the switch is switched on, the flyback converter stores energy in a magnetizing inductor shown at the primary side of the transformer. When the switch is switched off, the energy of the magnetizing inductor is transferred to the LED load of the secondary side of the transformer.
When the switch of the flyback converter is switched off, due to resonance by leakage inductance, a high-voltage spark may be generated. When the high-voltage spark leads the voltage between both terminals of the switch to rise so that the voltage between both terminals of the switch exceeds a rated voltage, the switch may be destroyed. To solve the problems, the flyback converter includes an RCD snubber circuit to consume the energy accumulated in the leakage inductance, so that the switch peak voltage may be restricted.
FIG. 2 is a circuit diagram showing a flyback converter. FIG. 3 is a waveform diagram illustrating a voltage between both terminals of the switch according to an on/off operation of the switch of the flyback converter.
The flyback converter 20 includes a snubber circuit 21. The RCD snubber circuit 21 for clamping includes a resistor R, a capacitor C and a diode D.
When the switch S of the flyback converter 20 is switched off, the voltage Vc applied to both terminals of the capacitor C of the snubber circuit 21 is equal to the sum of the component V1 generated by the secondary winding and the turn ratio and the component V2 accumulated in the leakage inductance, and the sum of the input voltage Vi and the voltage Vc applied to both terminals of the capacitor C of the snubber circuit 21 is applied to both terminals of the switch Q. Thus, the energy accumulated in the leakage inductance is transferred to the capacitor C of the snubber circuit 21, so that the energy may be consumed by the resistor R.
A surge voltage applied to both terminals of the switch may be restricted through the above-described principle. However, according to the related art, since the energy accumulated in the leakage inductance is consumed by the resistor R in the snubber circuit 21, power conversion efficiency is deteriorated. In addition, since the energy accumulated in the capacitor C of the snubber circuit 21 is consumed in the resistor R, the loss of the resistor R is increased in proportion to a switching frequency of the switch.
Until now, the flyback converter of the buck-boost type having the snubber circuit 21 is mainly described, bur likewise, the above described problems may occur in a forward converter of a buck type.